This invention relates to pressure sensors, and more particularly, to high accuracy, silicon capacitive pressure sensors utilizing a separate reference element.
In high accuracy (0.05% or 500 ppm) pressure sensing applications, long-term drift (20 years) at high temperatures (120.degree. C.) limits the overall achievable accuracy of a silicon capacitive pressure sensor. This limiting factor may make some sensor designs unsuitable for demanding aerospace applications, such as electronic engine controls ("EECs") and air data computers ("ADCs"). Each component of the sensing element package contributes a small, yet significant, amount to the total error budget of the sensor.
Prior art single element silicon capacitive pressure sensors typically comprise a sensing element made up of a pair of parallel conductive silicon plates, joined together with an insulative borosilicate glass spacer by a field-assisted, vacuum bonding process. This forms an evacuated capsule with opposing conductive surfaces. The opposing silicon pieces form the plates of a pressure variable capacitor. See, for example, U.S. Pat. Nos. 4,415,948, 4,405,970 and 4,530,029. Examples of electronic signal processing circuitry used to process the output signals indicative of sensed pressure from these sensors are described and claimed in U.S. Pat. Nos. 4,743,836 and 4,517,622.
One conductive silicon plate of the capacitive pressure sensor forms a diaphragm that flexes inwardly in the presence of fluid pressure applied to the outside surface of the diaphragm that is greater in magnitude than the pressure (usually vacuum) between the plates. The second conductive silicon plate forms a substrate that is normally held rigid. The deflection of the diaphragm causes a variation in the distance between the plates, thus varying the capacitance of the plates. Thus, pressure variations are transduced to capacitance variations in a typical silicon capacitive pressure sensor. The borosilicate glass serves not only to separate the plates but also to seal the vacuum enclosure therebetween. The diaphragm and substrate are normally doped to make them electrically conductive.
These pressure sensing devices are particularly well suited for miniaturization due to the fine dimensional control achievable using the semiconductor and thin film technologies. They are also well suited to the measurement of small differential pressures in various commercial and aerospace applications. Microcircuit technology can produce a large number of pressure sensors fabricated from a silicon wafer.
In any silicon capacitive pressure sensor, parasitic capacitance is a limitation on the accuracy of the sensor because it can result in an overall long-term drift. Parasitic capacitance is the inherent capacitance of the non-pressure sensitive interstices of the sensor structure. For example, the parasitic capacitance provided by the borosilicate glass spacer may comprise upwards of fifty percent (50%) of the total capacitance of the sensing element. Such parasitic capacitance reduces the gain of the pressure dependent capacitive sensor because it adds in parallel to the pressure-sensitive capacitance of the sensor. This reduces the dynamic range of the sensor and reduces its sensitivity to pressure changes. Also, the aging or drift in the electrical properties of the dielectric wall spacer has been identified as the major contributing factor to the drift of the sensing element. Thus, a large effort has been placed in the past on reducing such capacitance through variations in the design of the sensor architecture.
However, parasitic capacitance is inherent in any physical structure and there is a minimum practically achievable value that may still be unacceptable in high sensitivity sensing applications. The aforementioned U.S. Pat. No. 4,405,970 discloses a method of reducing the parasitic capacitance in a silicon capacitive pressure sensor by providing specific borosilicate glass structures that separate fixed portions of the two capacitive plates at a relatively long distance from each other. Another approach to reducing the parasitic capacitance is disclosed in U.S. Pat. No. 4,467,394, in which a three plate device is utilized that, when combined with appropriate signal processing circuitry, eliminates the parasitic capacitance from the measurement and, thus, eliminates the resulting overall drift and instability of the sensor. A further approach to eliminating the parasitic capacitance is disclosed in U.S. Pat. No. 4,951,174.
It has been discovered experimentally that the aging of the deposited borosilicate glass dielectric spacer (which electrically isolates the diaphragm of the sensor from the base or substrate) is the primary contributor to sensing element drift and, thus, long-term stability of the sensor. This differs from the performance of many other types of capacitive pressure sensors, such as metal diaphragm devices, in which the physical creep or movement of the diaphragm and base significantly contributes to sensor drift. In an attempt to solve this problem, it is known to provide a silicon capacitive pressure sensor having a reference capacitor comprised of an "off-the-shelf" capacitor. The intent is to try to match the dielectric materials of the reference and sensing elements together. However, this approach does not provide the requisite high degree of matching of aging properties of the sensing and reference elements needed for high accuracy aerospace applications. Thus, heretofore, there has clearly been a lack of appreciation of the high degree to which this matching must be achieved and the performance improvement derived therefrom.
Accordingly, it is a primary object to the present invention to achieve long-term (20 years) performance stability of a silicon capacitive pressure sensor by structurally matching to high precision the sensing and reference elements of the sensor, especially over the entire dynamic operating envelope of the sensor.
It is a general object of the present invention to provide a silicon capacitive pressure sensor with separate sensing and reference elements that have their aging effects identically matched by fabricating both elements from the same wafer and exposing them to identical processing and mounting steps.
It is another object of the present invention to provide a silicon capacitive pressure sensor having high accuracy, high reliability, small size, light weight, low temperature sensitivity, high dynamic range, excellent long term stability and operability in rugged environments.
The above and other objects and advantages of this invention will become more readily apparent when the following description is read in conjunction with the accompanying drawings.